The DDM damping field seemed to do zip against the phasers of the two starships, yet for some reason neither of the ships was seen attempting to use their photon torpedoes. It was also told the DDM "neutralized antimatter" in some manner that was a mystery even to Scotty. This would nicely account for the inability to use photon torpedoes and the difficulty of using warp to defeat the clumsy opponent.
Now, should we assume that the NOMAD weapon was comparable to photon torpedoes and therefore especially susceptible to DDM jamming? Our heroes did seem to want to compare the weapon to photon torpedoes specifically, rather than, say, to phasers or some abstract unit of power output. Obviously, this was for reasons of drama (counting photon torpedoes is audience-friendly, speaking of terawatts is not), but still...
As for the ability of NOMAD to assimilate DDM, I think that train already sailed. Tan Ru must have been something special to be able to merge with a stupid old Earth probe in a "collision" situation; NOMAD itself did not look like a good candidate for such special powers. Sure, it's nice to be able to interface wirelessly with alien computers and all, but Tan Ru appeared capable of more "physical" manipulation - it somehow fundamentally altered the appearance of NOMAD, after all.